Ultrafast Electron Dynamics in Coupled and Uncoupled HgTe Quantum Dots

In this article, we study electron dynamics in HgTe quantum dots with a 1.9 μm gap, a material relevant for infrared sensing and emission, using ultrafast spectroscopy with 35 fs time resolution. Experiments have been carried out at several probing photon energies around the gap, which allows us to follow the relaxation path of the photoexcited electrons. We compare such dynamics in two kind of samples, HgTe quantum dots with long ligands and with short ligands, in order to distinguish the role of the coupling between adjacent quantum dots. Three main dynamics can be observed in the transient reflectivity on both samples, with slightly different relaxation times: two fast decays on the time scale of hundreds of femtoseconds and a few picoseconds, respectively, followed by a slower relaxation back to the unperturbed value over hundreds of picoseconds. The two fast components are associated with intraband relaxation of the photoexcited electrons within the conduction band, while the final relaxation path can be assigned to Auger relaxation mechanisms and to the slower interband exciton recombination.